An assembly of the above-mentioned kind is schematically illustrated in FIG. 1, where 1 generally designates a containment for a nuclear power reactor 2. The containment has the shape of a cylindrical, often metal-lined concrete wall which is terminated at the top by a ceiling 3 and at the bottom by a bottom 4. An internal partition wall 5 near the reactor 2 delimits a ring-formed space, the bottom part of which forms a pool 6 for water. The water level is schematically indicated at 7. The internal partition wall 5 is terminated at the top by a circumfering ceiling part 8. In the space above said ceiling part 8, a plurality of line pipes 9 are mounted for feeding steam from the reactor 2, which in practice may consist of a boiling reactor.
In the event steam would leak out from the line pipes 9, e.g. as a consequence of unforeseen breakdowns, a particular steam blowing assembly is arranged in the containment, which assembly consists of a plurality of tubes 10, the upper and lower ends of which are open and which hang down from the ceiling part 8, more precisely immersed in the water of the pool 6. In practice, the water depth in the pool may attain 6-8 meters, the tubes 10 having a length of 12-15 meters so that the lower ends thereof are located close to the bottom of the pool. In practice, the number of tubes 10 in the assembly may amount to 30-90, each individual tube having a diameter of 50-60 cm.
In the event steam would come out in the top part of the containment, it may be blown down through the tubes 10 so as to be condensed as fast as possible in the water in the pool. Tests made have shown that steam which in this way rushes down through the individual tube under pressure applies considerable mechanical stresses to the tube as a consequence of explosion-like shocks arising when the steam leaves the lower opening of the tube, at the same time as the water in the pool rises or heaves. During a certain phase, steam may be brought out in the pool and to a certain extent condense therein, but also to a certain extent cause the water to rise, the water pressure in the bottom part of the pool increasing, more precisely to a level at which the water pressure increases above the steam pressure, water in a second phase beginning to rise up in the blow down tubes while lowering the water level in the pool. Then implosion-like shock phenomena arise in the area of the lower, open ends of the tubes. After this, the phases may vary in such a way that steam in one moment rushes down through the tubes and out in the pool water and pool water, in another moment, rises up in the tubes. By the fact that the blow down tubes are often fastened only at the upper ends thereof, more precisely in the ceiling part 8, the above-mentioned shock stresses on the lower, free-hanging ends of the tubes imply a security risk inasmuch as the tubes may be damaged or even come loose under extreme circumstances.
In order to counteract this risk, tests have been made to strap the lower parts of the tubes by means of wires and the like. However, wires which are mounted in the pool are exposed to very large stresses when the water heaves, and therefore this solution is not reliable. Furthermore, tests have been made to modify the shape of the lower ends of the blow down tubes so that shock phenomena are mitigated. Among other things, the lower end of the tube has been obliquely cut. Then, however, considerable transverse loads on the lower part of the tubes arise in connection with the shocks. Also, the lower end of the tube has been made with a cross-section-wise semi-circular corbelling around which the steam and the water respectively may move more smoothly than around a sharp tube wall edge. However, nor these tests have been successful, although a certain improving effect has been noted.